Question 1. What is dark matter?
Astronomers have shown that the objects in the universe, from galaxies millions of times smaller than ours to the largest clusters of galaxies, are held together by a form of matter different from what we are made of and that gives off no light. This matter probably consists of one or more as-yet-undiscovered elementary particles, and aggregations of it produce the gravitational pull leading to the formation of galaxies and large-scale structures in the universe. At the same time these particles may be streaming through our Earth-bound laboratories.
What is the evidence that dark matter exists?
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In the 1930’s, while measuring the velocity and rotation of clusters of galaxies, astronomers discovered they were moving faster than expected due to gravity. Either they couldn’t see all the mass, or our understanding of gravity is wrong. Scientists believed they understood gravity, so they dubbed this the ‘missing mass’ problem. In more recent years, two observations have convinced scientists that dark matter must not only exist but must constitute 23% of the universe. One of these is the red-shift of galaxies, and the other is gravitational lensing. |
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In red-shift, astronomers use the frequency shift of spectral lines to determine how fast it is moving. This is analogous to Doppler shift in sound. |
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Einstein predicted that light would bend around an object causing a ring event. He called this gravitational lensing. If there is no detectable mass between the object and the observer, a ring must be caused by mass that we can’t detect, or dark matter. |
What could dark matter be?
All matter is subject to forces, which determine how it interacts with our eyes, telescopes, bodies and large detectors we might put underground. If we cannot ‘see’ dark matter, we know it cannot be subject to our two strongest forces, electromagnetic and strong nuclear. We know it does interact with gravity, since we see the rings due to gravitational lensing. We do not know whether it is subject to the weak nuclear force, but it is our strongest hope for seeing it.
| Gravity | Electro- magnetism |
Weak nuclear | Strong nuclear | |
| Ordinary matter | X | X | X | X |
| Photons | X | X | ||
| Neutrinos | X | X | ||
| Dark matter | X | X? |
Most experiments being designed to look for dark matter assume it is subject to the weak force and call the particles they are looking for, Weakly Interacting Massive Particles, or WIMPs. They act like neutrinos, then, but are thought to be much more massive, in order to account for all the mass missing in the universe.
How are scientists trying to detect it in DUSEL?
If dark matter is made up of unidentified particles, our solar system and the Earth would be passing through a sea of dark matter particles that constitute a dark halo of the Milky Way galaxy. These particles might be directly measured, but the chance that a WIMP might bump into an atom and knock it enough to be detectable is extremely small. Estimates range from once a day to less than once per century for kilogram of target material. This requires going deep underground to get away from other backgrounds such as cosmic rays. Several dark matter searches are proposed for DUSEL using various kinds of target material. The Large Underground Xenon (LUX) experiment will begin running in 2010 at the Sanford Underground Laboratory at Homestake.
More about Dark Matter (no particular order):
NASA Genesis mission educational material on dark matter
http://genesismission.jpl.nasa.gov/educate/scimodule/Cosmogony/CosmogonyPDF/
Dark%20MatterTG.pdf
Educational material on dark matter from the Cryogenic Dark Matter Search (CDMS) group at Univ. California Berkeley
http://cdms.berkeley.edu/Education/DMpages/index.shtml
‘The Mystery of Dark Matter’ is a one-two period lesson plan from the Perimeter Institute for Theoretical Physics in Canada.
*A DVD of the material can be ordered.
http://www.perimeterinstitute.ca/Perimeter_Explorations/The_Mystery_of_Dark_Matter/
The_Mystery_of_Dark_Matter/
NASA Goddard ‘Imagine the Universe’ material on dark matter
http://imagine.gsfc.nasa.gov/docs/science/know_l1/dark_matter.html
Gravitational lensing demonstration using a wine bottle and flashlight
http://doversherborn.org/~bridger/Astronomy/projects/radio/activities/demo/lensing/
lensing.htm
Gravitational lensing demonstrator using Adobe Photoshop
http://leo.astronomy.cz/grlens/grl0.html
Keep up to date with happenings at Sanford Underground Laboratory at Homestake by visiting http://www.sanfordundergroundlaboratoryathomestake.org/